The present invention relates to improvements in compression fittings especially adapted for use in the joining of thermoplastic pipe and fittings. In particular the invention relates to compression fittings for use with flexible thermoplastic pipe which is to be connected to rigid components of a piping system.
Recent years have seen the development to commercial production levels of thermoplastic materials having favorable heat distortion characteristics and resistance to chemical attack by water. Their properties have been such as to make them suitable in pipe and fitting form for application to hot and cold water supply in residential construction and to various uses in the food and chemical industries.
Certain of these thermoplastics are capable of being joined by solvent welding, that is, by the chemical action of solvents at the interface of pipes and fittings. In this category are such materials as chlorinated polyvinylchloride, or CPVC, and poly-phenylene oxide, or PPO. Other favorable high heat distortion thermoplastics such as polybutylene and cross linked polyethylene cannot readily be attacked by the chemical action of solvents, and so they are joined by mechanical means.
The high heat distortion thermoplastics that are capable of being joined by solvent welding are relatively rigid materials. On the other hand, those that necessitate joining by mechanical means are relatively flexible.
Special problems arise in joining the flexible materials to the rigid materials, although both are thermoplastics. This is also true of joining the flexible thermoplastics to other rigid materials such as copper and iron. The problems are particularly aggravated when elevated temperatures of operation are involved, and are also a matter of concern at lower temperatures. Whereas CPVC is relatively hard, and resists cutting by sharp edges, polybutylene can be cut with a knife when cold, and as it is elevated in temperature it becomes progressively more flexible and easy to cut. The problem then is to secure the polybutylene pipe against end stress, to get a grip on it sufficiently to retain it in the fitting, up to at least the fail point of the pipe itself.
Efforts have been made to use compression fitting for thermo-plastic pipe, but none of these have proved to be a satisfactory solution to the problems set forth above, because of their tendencies to cut or abrade the flexible pipe, lack of holding capability, inadequate sealing capacity, or the like. A pipe coupling such as is disclosed in U.S. Pat. No. 3,563,575, issued Feb. 16, 1971 to Sanford discloses a pipe coupling which would be unsuitable for the desired purpose, because the pipe coupling would not provide adequate holding to overcome end stress or to assure a leak proof joint. A pipe connector or tube coupling such as are disclosed in U.S. Pat. Nos. 3,312,483, and 3,312,484, issued Apr. 4, 1967 to Leadbetter, et al., and Davenport, respectively, show arrangements which are objectionable because the flexible pipe is subject to cutting or abrading action by the connector or coupling.